#include <stdio.h>

#define MAX 400

int main()
{
    int n, tmp, maxx = 0, minx = 0, ans = 0;
    int a[MAX][2], x[110], y[110];
    scanf("%d", &n);
    for (int i = 0; i < n; i++)
    {
        scanf("%d %d", &x[i], &y[i]);
    }
    x[n]=x[0];
    y[n]=y[0];
    for (int i = 0; i < n; i++)
    {
        ans += (x[i] * y[i + 1] - x[i + 1] * y[i]);
    }

    printf("%d", ans/2);

    return 0;
}
